In recent years, with a view to saving energy, a lamp in which a semiconductor light emitting element including an LED (Light Emitting Diode) is used as a light source has been developed as a bulb type lamp for replacing an incandescent bulb. For example, Japanese Unexamined Patent Application Publication Nos. 2008-227412 and 2011-82141 disclose a bulb type lamp that includes a light emitting module formed by mounting a semiconductor light emitting element to a mounting substrate, a base for supporting the light emitting module, a circuit unit, a circuit holder, a metal cap and a globe.
The base serves as a heat sink that dissipates heat generated from the semiconductor light emitting element. In the lamp disclosed in Japanese Unexamined Patent Application Publication No. 2008-227412, a substrate having an LED is placed on a mounting surface of an aluminum-made base and is fixed to the base by tightening a screw. In this lamp, a ceramic substrate is often used as a module substrate that constitutes a light emitting module.
The ceramic substrate is made of a ceramic material such as an aluminum oxide, an aluminum nitride or a silicon nitride. The ceramic substrate is good in insulation property and is superior in heat dissipation property and heat resistance.
However, the ceramic-made module substrate is more brittle and fragile than a metal-made substrate. For that reason, if the ceramic-made module substrate is pressed against and fixed to the base by tightening a screw, it is sometimes the case that a stress is generated in a pressed point and a crack occurs.
Furthermore, during the use of a light emitting device, it is sometimes the case that, due to a difference in thermal expansion coefficient between a ceramic substrate and a main body, a stress is generated in the portion of the ceramic substrate pressed against a base and the ceramic substrate is broken (see paragraph 0005 of Japanese Unexamined Patent Application Publication No. 2011-82141).
The generation of a crack in the module substrate may be a cause of lighting failure. Thus, in the illumination device disclosed in Japanese Unexamined Patent Application Publication No. 2011-82141, the ceramic substrate is fixed to the base through a pressing member in a uniformly pressed state. This makes it possible to suppress breakage of the ceramic substrate while firmly fixing the ceramic substrate to the base.
In this case, however, the pressing member needs to be used. It is therefore desirable to suppress breakage of the module substrate with a simplified configuration. Moreover, even when the module substrate is pressed using the pressing member, a stress is generated in the module substrate. In particular, if the module substrate is tightly fastened at two or more points, a stress is generated and a crack is likely to occur.